Steer-by-wire steering systems comprising an individual electric actuator per steered wheel are known, the steering system being capable of turning each of the steered wheels selectively by an angle specific to it, the consistency of the steering angles of each of the steered wheels being ensured by the control electronics. The electric actuator for each wheel has the purpose of imparting the steering angle chosen by the control electronics to the wheel in question. The steering control available to the driver of the vehicle may be a conventional steering wheel or a lever of the joystick type or any other suitable device. The commands imparted by the driver of the vehicle on his control device are sent to the control electronics which are loaded with suitable programs so that they can control the actuator or actuators appropriately.
One of the advantages of this technology is that it is an ideal alliance of electronics and computing, progress in which areas is allowing increasingly sophisticated control systems, which means that it is possible to place the steering of the wheels not only under the control of the manual control but also under the control of a safety system. Thus, for example, the steered wheels can be set at an angle which not only takes account of the commands input by the driver of the vehicle but which also takes account of dynamic parameters observed on the vehicle.
Thanks to the steer-by-wire technology, a far wider range of possibilities becomes available for influencing the course stability of a vehicle. For example, whereas hitherto an automatic vehicle course-correction system has imparted corrective yaw moments through the brakes of one or more wheels, the switch to electrical control of the various functions in a vehicle would allow the steering angle of the various steered wheels of the vehicle to be used to correct the course of this vehicle.
However, the steering system of a vehicle is a function that is essential and vital to safety, as are the brakes. Hence, in order to be able to take the place of power-assisted or unassisted mechanical steering systems which are almost universally adopted at the present time on all road-going vehicles, it is essential that a steer-by-wire system be extremely reliable. This is why redundant electrical systems are generally designed as these allow the system to operate even if one of its components fails. This is the idea behind the breakdown-tolerant system. All the functionalities are performed if one of the components of the system, or at least one of the components deemed to be the most important of the system, fails. Thus, for example, the electrical parts of the wheel actuators are preferably redundant. Patent application US 2003/0098197 which provides an example of a redundant system for controlling steer-by-wire system may, for example, be consulted on this subject.
Patent U.S. Pat. No. 5,014,802 describes a steering system for a 4-wheel steering vehicle which proposes that, when a first steered wheel reaches the end of its steering travel and the driver wishes to increase the steering angle still further, the Ackerman steering be followed so as to prevent the wheels from skidding along the ground under all steering circumstances. Even though the inventors have anticipated applying the invention to a system in which all the wheels are controlled individually, the possibility of a failure and the strategies to be adopted in such are not part of this description.
Now, the failure of an essential component can never be completely excluded. For example, an electric wheel actuator may fail in such a way that it adopts some arbitrary angular position, in which situation it is no longer able to transmit steering forces (or forces for holding a straight line). Alternatively, it may remain locked in a certain angular position, for example in a straight line, as a result of a mechanical problem. It is the event of an actuator locking up that this patent application addresses.
Among the known solutions, patent application US 2004/0140147 proposes to control the other wheel actuator on the same axle to a modified reference angular position so that, on the whole, the axle develops a transverse thrust that differs as little as possible from the thrust that would have been obtained without the actuator failure. This solution has the disadvantage of introducing opposing stresses, particularly in the tyres. It is therefore desirable to adopt this approach only as a last resort. There is still a need to find solutions to deal with an electric actuator locking up without developing such opposing forces.